Scanning microscopy techniques, including near-field and confocal scanning microscopy, conventionally employ a single spatially localized detection or excitation element, sometimes known as the scanning probe [see, e.g., xe2x80x9cNear-field Optics: Theory, Instrumentation, and Applications,xe2x80x9d M. A. Paesler and P. J. Moyer, (Wiley-New York) (1996); xe2x80x9cConfocal Laser Scanning Microscopy,xe2x80x9d C. Sheppard, BIOS (Scientific-Oxford and Springer-New York) (1997).] The near-field scanning probe is typically a sub-wavelength aperture positioned in close proximity to a sample; in this way, sub-wavelength spatial resolution in the object-plane is obtained. The sub-wavelength aperture is an aperture smaller than the free-space optical wavelength of the optical beam used in the near-field microscopy application. Spatially extended images, e.g., two dimensional images, are acquired by driving the scanning probe in a raster pattern.
The invention features systems and methods that incorporate interferometric techniques into near-field microscopy. The near-field aspects of the system provide high spatial resolution and the interferometric techniques enhance the signal-to-noise ratio. Moreover, the systems and methods can further incorporate confocal microscopy techniques to further enhance the signal-to-noise ratio. The systems and methods can operate in reflection or transmission mode, and can be used to study surface properties of an unknown sample, to inspect a sample, such as microlithography mask or reticle, and to read information from, and/or write information to, and optical storage medium.
The systems and methods produce a near-field probe beam by illuminating a mask having an aperture with a dimension smaller than the free-space wavelength of the illuminating beam. The near-field probe beam interacts with a sample to produce a near-field signal beam, which is subsequently mixed with a reference beam to produce an interference signal. The properties of the near-field probe beam, such as its electric field and magnetic field multipole expansions, and their resulting interaction with the sample can be controlled by varying the incident angle of the illuminating beam, varying the distance between the mask and the sample, and tailoring the properties of the aperture. Furthermore, in some embodiments, the polarization and wavelength of the near-field probe beam can be varied.
The interference signal produced by interfering the near-field signal beam with a reference beam increases the near-field signal because the resulting signal scales with the amplitude of the near-field signal beam rather than its intensity. Furthermore, the changes in the phase and/or amplitude of the near-field signal beam, and the surface information corresponding to such changes, can be derived from the interference signal as the near-field probe beam scans the sample. Moreover, background contributions to the interference signal can be suppressed by introducing multiple phase shifts between the reference beam and the near-field probe beam and analyzing the interference signals as a function of the phase shifts. The phase shifts can be introduced, e.g., by using a phase-shifter or by introducing a difference frequency in components of input beam used to produce the near-field probe beam and the reference beam. In many embodiments, the phase shifting techniques are used in conjunction with a mask further having a non-transmissive scattering site adjacent the aperture, with the systems and methods producing an interference signal derived from mixing the light scattered from the scattering site with the reference beam to provide information about background signals that may be present in the interference signal derived from the near-field signal beam.
In further embodiments, the mask includes an array of apertures to direct an array of near-field probe beams to different locations on the sample, and the methods and systems produce a corresponding array of interference signals to more rapidly analyze the sample.
One embodiment of the invention can be generally described as follows.
An input beam including a linearly polarized single frequency laser beam is incident on a beam splitter. A first portion of the input beam is transmitted by the beam splitter as a measurement beam. A first portion of the measurement beam is incident on a sub-wavelength aperture in a conducting surface and a first portion thereof is transmitted as a near-field probe beam. The wavelength referenced in the sub-wavelength classification of the size of the sub-wavelength aperture is the wavelength of the input beam. A portion of the near-field probe beam is reflected and/or scattered by an object material back to the sub-wavelength aperture and a portion thereof is transmitted by the sub-wavelength aperture as a near-field return probe beam (i.e., the near-field signal beam). A second portion of the measurement beam incident on the sub-wavelength aperture is scattered by the sub-wavelength aperture as a first background return beam. The near-field return probe beam and the first background return beam comprise a return beam.
A second portion of the measurement beam is incident on a sub-wavelength non-transmitting scattering site located on the conductor at a position laterally displaced from the sub-wavelength aperture in the conductor by a preselected distance. The preselected distance is greater than or of the order of the wavelength of the input beam. A portion of the measurement beam incident on the sub-wavelength scattering site is scattered as a second background return beam.
A second portion of the input beam is reflected by the beam splitter as a reference beam. The reference beam is incident on a reference object and reflected as a reflected reference beam.
The return beam and a portion of the reflected reference beam are incident on the beam splitter and mixed by a polarizer as a first mixed beam. The first mixed beam is then focused on a pinhole in an image plane such that an image of a sub-wavelength aperture is in focus in the plane of the pinhole. The size of the preselected separation of the sub-wavelength aperture and the sub-wavelength scattering site, the size of the pinhole, and the resolution of an imaging system producing the image of the sub-wavelength aperture on the pinhole are selected such that a substantially reduced portion of the second background return beam is transmitted by the pinhole. A portion of the focused first mixed beam is transmitted by the pinhole and detected, preferably by a quantum photon detector [see Section 15.3 in Chapter 15 entitled xe2x80x9cQuantum Detectorsxe2x80x9d, Handbook of Optics, 1, 1995 (McGraw-Hill, New York) by P. R. Norton], to generate a first electrical interference signal. The amplitude and phase of the first electrical interference signal is measured.
The second background return beam and a second portion of the reflected reference beam are incident on the beam splitter, mixed by a polarizer, and the second mixed beam is then focused on a second pinhole such that the sub-wavelength scattering site is in focus in the plane of the second pinhole. The size of the preselected separation of the sub-wavelength aperture and the sub-wavelength scattering site, the size of the second pinhole, and the resolution of an imaging system producing the image of the sub-wavelength scattering site on the second pinhole are selected such that a substantially reduced portion the return beam is transmitted by the second pinhole. A portion of the focused second mixed beam is transmitted by the second pinhole and detected, preferably by a quantum photon detector, to generate a second electrical interference signal.
The amplitude and phase of the second interference signal are measured and the measured amplitude and phase of the second interference signal is used to compensate for the effects of the first background return beam on the measured amplitude and phase of the first electrical interference signal.
The measured amplitude and phase of the compensated first electrical interference signal are analyzed for the determination of the amplitude and phase of the near-field return probe beam. Next, the object is scanned and a resulting array of determined amplitudes and phases of near-field return probe beam are obtained and subsequently analyzed for information about the distance between the sub-wavelength aperture in the conductor and the object material and for information about the structure of the object material.
The scanning of the object can be in either a step and repeat mode or a continuous mode. For a continuous scanning mode, the source is preferably pulsed.
In further embodiments, the sub-wavelength aperture, the sub-wavelength scattering site, the first pinhole, the second pinhole, and the detector are replaced by arrays of sub-wavelength apertures, sub-wavelength scattering sites, first pinholes, second pinholes, and detector pixels. In certain other embodiments, the measurement beam is incident on the sub-wavelength aperture(s) and sub-wavelength scattering site(s) at a large angle of incidence.
In further embodiment, the sample object is an optical recording medium. For example, the object material can comprise a magneto-optical material, and the near-field return probe beam is measured to determine the state of magnetization of magneto-optical domains comprising the magneto-optical material used as the optical recording medium. Data are stored on the magneto-optical material by locally heating the magneto-optical material with near-field probe beams in the presence a magnetic field.
Aspects and features disclosed in the following, commonly-owned provisional applications may also be incorporated into the embodiments described in the present application: Serial No. 09/917,399 filed Jul. 27, 2000 by Henry A. Hill entitled xe2x80x9cScanning interferometric near-field confocal microscopy with background amplitude reduction and compensation;xe2x80x9d Serial No. 09/917,402 filed Jul. 27, 2000 by Henry A. Hill and Kyle B. Ferrio entitled xe2x80x9cMultiple-Source Arrays For Confocal And Near-Field Microscopy;xe2x80x9d Serial No. 09/917,400 filed Jul. 27, 2000 by Henry A. Hill entitled xe2x80x9cMultiple-Source Arrays with Optical Transmission Enhanced by Resonant Cavities;xe2x80x9d U.S. Serial No 09/917,401 by Henry A. Hill filed Jul. 27, 2000 entitled xe2x80x9cControl of Position and Orientation of Sub-Wavelength Aperture Array in Near-field Scanning Microscopy;xe2x80x9d and U.S. Serial No. 09/917,276 by Henry A. Hill filed Jul. 27, 2000 entitled xe2x80x9cDifferential Interferometric Confocal Near-Field Microscopy;xe2x80x9d the contents of each of the preceding provisional applications being incorporated herein by reference.
In general, in one aspect, the invention features a near-field, interferometric optical microscopy system. The system includes: a beam splitter positioned to separate an input beam into a measurement beam and a reference beam; a mask positioned to receive the measurement beam, the mask including at least one aperture having a dimension smaller than the wavelength of the input beam, wherein the mask aperture is configured to couple at least a portion of the measurement beam to a sample to define a near-field probe beam, the sample interacting with the near-field probe beam to define a near-field signal beam; a detector having an element responsive to optical energy; and optics positioned to direct at least a portion of the reference beam and at least a portion of the near-field signal beam to interfere at the detector element.
Embodiments of the microscopy system may include any of the following features.
The optics and the beam splitter can be positioned to direct the at least a portion of the reference beam and the at least a portion of the near-field signal beam to interfere at the detector element.
The sample can transmit a portion of the near-field probe beam to define the near-field signal beam. The system can further include a second mask positioned to receive the near-field signal beam, the second mask including at least one aperture having a dimension smaller than the wavelength of the input beam, wherein the aperture in the second mask is configured to couple the near-field signal beam to the optics. The optics can include a spatial filter positioned before the detector, the spatial filter including a pinhole aligned with the detector element, and imaging optics positioned to image at least a portion of the near-field signal beam onto the pinhole. For example, the portion of the near-field signal beam imaged onto the pinhole can be that emerging from the aperture in the second mask.
Alternatively, the sample can scatter at least a portion of the near-field probe beam to define the near-field signal beam, and the mask aperture can be configured to couple the near-field signal beam to the optics. Furthermore, the mask aperture can scatter another portion of the measurement beam to define a background return beam, and the optics can direct the at least a portion of the reference beam, the at least a portion of the near-field signal beam, and at least a portion of the background return beam to interfere at the detector element. In such a case, the mask can further include a scattering site adjacent the aperture, the scattering site having a dimension smaller than the wavelength of the input beam. During operation the scattering site scatters an additional portion of the measurement beam to define a second background return beam, the detector includes a second element responsive to optical energy, and the optics are positioned to direct another portion of the reference beam and at least a portion of the second background return beam to interfere at the second detector element.
Furthermore, the optics can include a spatial filter positioned before the detector, the spatial filter including a pinhole aligned with the detector element, and imaging optics positioned to image at least a portion of the near-field signal beam emerging from the aperture onto the pinhole. Where the mask includes a scattering site, the optics can include a spatial filter positioned before the detector, the spatial filter including a first pinhole aligned with the first detector element and a second pinhole aligned with the second detector element, and imaging optics positioned to image at least a portion of the near-field signal beam emerging from the aperture onto the first pinhole and at least a portion of the second background return beam emerging from the scattering site onto the second pinhole. The optics can further include a reference object positioned to redirect the reference beam towards the detector, the reference object having first and second reflective sites each having a dimension smaller than the wavelength of the input beam. The imaging optics then further image a first portion of the reference beam reflected by the first reflective site onto the first pinhole and a second portion of the reference beam reflected by the second reflective site onto the second pinhole.
The mask can include a plurality of apertures each having a dimension smaller than the wavelength of the input beam, wherein each aperture is configured to couple a portion of the measurement beam to the sample to define a near-field probe beam for the aperture, the sample interacting with the near-field probe beams to define corresponding near-field signal beams. In such a case, the detector includes a plurality of elements each responsive to optical energy, each near-field signal beam having a corresponding detector element, and the optics direct at least a portion of each near-field signal beam and a portion of the reference beam to interfere at the corresponding detector element.
The mask can include a plurality of apertures each having a dimension smaller than the wavelength of the input beam, wherein each aperture is configured to couple a portion of the measurement beam to the sample to define a probe beam for the aperture, scatter a another portion of the measurement beam to define a background return beam for the aperture, and couples at least a portion of the probe beam scattered by the sample back through itself to define a near-field-signal beam for the aperture. In such a case, the mask can further include a plurality of scattering sites each having a dimension smaller than the wavelength of the input beam, each scattering site being adjacent to one of the apertures, wherein each scattering site is configured to scatter a portion of the measurement beam to define a second background return beam. The detector then includes a plurality of elements each responsive to optical energy, each near-field signal beam having a corresponding detector element and each background return beam having another corresponding detector element, and the optics direct at least a portion of each near-field signal beam and a portion of the reference beam to interfere at the corresponding detector element and at least a portion of each background return beam and another portion of the reference beam to interfere at the other corresponding detector element.
The optics in the microscopy system can define a confocal imaging system.
The system can further include a stage for supporting the sample and at least one of a scanner and a stepper coupled to the stage for adjusting the position of the sample relative to the near-field probe beam. The system can further include a electronic processor coupled to the detector and the at least one of the scanner and the stepper, wherein during operation the electronic processor analyzes at least one signal generated by the detector element as a function of the relative stage position. The system can further include a pulsed source, which during operation generates the input beam, wherein the electronic processor is coupled to the pulse source to synchronize the stage adjustments.
The beam splitter and the mask can be positioned to cause the measurement beam to contact the mask at substantially normal incidence. Alternatively, the beam splitter and the mask can be positioned to cause the measurement beam to contact the mask at an angle of incidence greater than 10xc2x0.
The aperture can be defined by a hole in the mask. Alternatively, the mask can include a first material having a first complex refractive index and a second material having a second complex refractive index different from the first complex refractive index, the second material defining the aperture. Also, the mask can include a waveguide defining the aperture. Also, the mask can include a first reflective material and a second dielectric material defining the aperture. Also, where the mask includes a scattering site, the mask can include a reflective first material and a second material having optical properties different from those of the first material, the second material defining the scattering site.
The system can further include a phase shifter positioned to shift the phase of the reference beam relative to the phase of the measurement beam. For example, the phase shifter can be positioned along the path of the reference beam. The system can further include a electronic processor coupled to the detector and the phase shifter, wherein during operation the electronic processor sets the phase shift imparted by the phase shifter to each of multiple values and analyzes a signal generated by the detector element for each of the multiple values.
Where the mask includes at least one aperture and at least one scatter site, the system can further include a phase shifter positioned to shift the phase of the reference beam relative to the phase of the measurement beam and a electronic processor coupled to the detector and the phase shifter, wherein during operation the electronic processor sets the phase shift imparted by the phase shifter to each of one of multiple values and analyzes a signal generated by each of the first and second detector elements for each of the multiple values. For example, the multiple phase shift values can include at least four phase shift values, such as values corresponding to about "khgr"0, "khgr"0+xcfx80, "khgr"0+xcfx80/2, and "khgr"0+3xcfx80/2 radians, where "khgr"0 is any constant value. In such a case, for each of the first and second detector elements, the analyzer determines a first difference between the signals corresponding to the phase shift values "khgr"0 and "khgr"0+xcfx80 and a second difference between the signals corresponding to the phase shift values "khgr"0+xcfx80/2 and "khgr"0+3xcfx80/2. The electronic processor can determine a complex amplitude for the near-field signal beam based on the first and second difference signals for each of the detector elements. Furthermore, the analyzer can use the complex amplitude of the near-field signal beam to derive a physical property of the sample at the location illuminated by the probe beam.
In further embodiments including a phase shifter, the system can further include a electronic processor coupled to the detector and the phase shifter, wherein during operation the electronic processor causes the phase shift imparted by the phase shifter to be modulated according to "khgr"="khgr"0+"khgr"1 cos xcfx89t, where "khgr"1≈0, t is time, and xcfx89 is the modulation frequency, and analyzes a signal generated by the detector element with respect to the modulation frequency.
The system can further include a second detector having an element responsive to optical energy, wherein at least one of the optics and the beam splitter is positioned to direct another portion of the reference beam and another portion of the near-field signal beam to interfere at the detector element of the second detector. In such a case, the first and second detectors can define first and second detection channels for the system. The system can further include a first phase shifter positioned to shift the phase of the reference beam relative to the phase of the measurement beam; a second phase shifter positioned to shift the phase of the other portion of the reference beam relative to the other portion of the near-field signal beam; and an electronic processor coupled to the phase shifters.
The system can further include a source for generating the input beam. Furthermore, the source can include a modulator producing a frequency difference xcfx89 between two components of the input beam, the frequency difference xcfx89 producing a phase difference xcfx89t between the two components of the input beam, where t is time. The system can further include an electronic processor coupled to the detector and the modulator, wherein the electronic processor analyzes a signal produced by the detector with respect to the phase difference xcfx89t. Moreover, the source can be a pulse source, wherein the electronic processor is coupled to the pulsed source to synchronize the signal analysis with the phase difference xcfx89t. Furthermore, the source can cause the input beam to have one of multiple wavelengths. In such a case, the system can further include an electronic processor coupled to the detector and the source, wherein the electronic processor analyzes a signal produced by the detector for each of the multiple wavelengths of the input beam.
The system can further include a retardation plate positioned along the path of the input beam and configured to adjustably control the polarization of the input beam. In such a case, the system can further include an electronic processor coupled to the detector and the retardation plate, wherein during operation the electronic processor causes the retardation plate to impart each of multiple polarizations to the input beam and analyzes a signal generated by the detector element for each of the multiple polarizations.
In another aspect, the invention features a lithography system for use in fabricating integrated circuits on a wafer having alignment marks. The system includes: a stage for supporting the wafer; an illumination system for imaging spatially patterned radiation onto the wafer; a positioning system for adjusting the position of the stage relative to the imaged radiation and the alignment marks; and the near-field, interferometric optical microscopy system described above coupled to the positioning system for identifying the position of the alignment marks on the wafer.
In another aspect, the invention features a beam writing system for use in fabricating a lithography mask. The system includes: a source providing a write beam to pattern a substrate; a stage supporting the substrate; a beam directing assembly for delivering the write beam to the substrate; a positioning system for positioning the stage and beam directing assembly relative one another; and the near-field, interferometric optical microscopy system described above for measuring the surface profile of the patterned substrate.
In another aspect, the invention features, a mask inspection system including: the near-field, interferometric optical microscopy system described above for measuring surface properties of a fabricated mask; and an electronic processing system coupled to the microscopy system, which during operation compares the surface properties of the fabricated mask to stored data. In one embodiment, the stored data is derived from data used to produce the fabricated mask. In another embodiment, the stored data is derived from measurement by the microscopy system of another fabricated mask.
In general, in another aspect, the invention features a microscopy method for measuring surface properties of a sample including: separating an input beam into a measurement beam and a reference beam; directing the measurement beam to a mask including at least one aperture having a dimension smaller than the wavelength of the input beam, wherein the mask aperture couples at least a portion of the measurement beam to the sample to define a near-field probe beam, the sample interacting with the near-field probe beam to define a near-field signal beam; and measuring optical interference between at least a portion of the reference beam and at least a portion of the near-field signal beam. The microscopy method can further include features corresponding to any of the features of the near-field, interferometric optical microscopy system described above.
In another aspect, the invention features an inspection method including using the microscopy method described above to measure surface properties of the sample; and comparing the surface properties to reference data. For example, the sample can be one of a mask, a reticle, and patterned wafer.
In general, in another aspect, the invention features an optical storage system including: a beam splitter positioned to separate an input beam into a measurement beam and a reference beam; a mask positioned to receive the measurement beam, the mask including at least one aperture having a dimension smaller than the wavelength of the input beam, wherein the mask aperture is configured to couple at least a portion of the measurement beam to an optical storage medium to define a near-field probe beam, the storage medium interacting with the near-field probe beam to define a near-field signal beam; a detector having an element responsive to optical energy; and optics positioned to direct at least a portion of the reference beam and at least a portion of the near-field signal beam to interfere at the detector element.
Embodiments of the optical storage system may include any of the following features.
The system can further include an electronic processor coupled to the detector, wherein during operation the electronic processor analyzes signals produced by the detector to determine a memory state of the optical storage medium at the location illuminated by the near-field probe beam.
The system can further include a second detector having an element responsive to optical energy, wherein the optics are positioned to direct a second portion of the reference beam and a second portion of the near-field signal beam to interfere at the element of the second detector. In such a case, the optics can cause the first-mentioned portion of the reference beam and the second portion of the reference beam to have different polarizations and the first-mentioned portion of the near-field signal beam and the second portion of the near-field signal beam to have different polarizations. For example, the optics can include a polarizing beam splitter positioned to separate the first-mentioned portion of the reference beam from the second portion of the reference beam and separate the first-mentioned portion of the near-field signal beam from second portion of the near-field signal beam. The optics can further include a retardation plate, e.g., a half-wave plate, the polarizing beam splitter being positioned between the retardation plate and the detectors.
Also, in embodiments having the first and second detectors, the system can further include an electronic processor coupled to the first-mentioned detector and the second detector, wherein during operation the electronic processor analyzes signals produced by the detectors to determine a memory state of the optical storage medium at the location illuminated by the near-field probe beam.
The system can further include the optical storage medium, wherein the optical storage medium includes multiple domains, at least some of the domains altering the polarization of an incident beam. For example, the optical storage material can be a magneto-optic material.
The system can further include the optical storage medium, wherein the optical storage medium has multiple domains, at least some of the domains defined by a variation in complex refractive index.
The optical storage medium can transmit a portion of the near-field probe beam to define the near-field signal beam. The system can further include a second mask positioned to receive the near-field signal beam, the second mask including at least one aperture having a dimension smaller than the wavelength of the input beam, wherein the aperture in the second mask is configured to couple the near-field signal beam to the optics. The optics can include a spatial filter positioned before the detector, the spatial filter including a pinhole aligned with the detector element, and imaging optics positioned to image at least a portion of the near-field signal beam onto the pinhole. For example, the portion of the near-field signal beam imaged onto the pinhole can be that emerging from the aperture in the second mask.
Alternatively, the optical storage medium can scatter at least a portion of the near-field probe beam to define the near-field signal beam, and the mask aperture can be configured to couple the near-field signal beam to the optics. In such a case, the optics and the beam splitter can be positioned to direct the at least a portion of the reference beam and the at least a portion of the near-field signal beam to interfere at the detector element.
Furthermore, the mask aperture can scatter another portion of the measurement beam to define a background return beam, and the optics can direct the at least a portion of the reference beam, the at least a portion of the near-field signal beam, and at least a portion of the background return beam to interfere at the detector element. In such a case, the mask can further include a scattering site adjacent the aperture, the scattering site having a dimension smaller than the wavelength of the input beam. During operation the scattering site scatters an additional portion of the measurement beam to define a second background return beam, the detector includes a second element responsive to optical energy, and the optics are positioned to direct another portion of the reference beam and at least a portion of the second background return beam to interfere at the second detector element.
The mask can include a plurality of apertures each having a dimension smaller than the wavelength of the input beam, wherein each aperture is configured to couple a portion of the measurement beam to the optical storage medium to define a near-field probe beam for the aperture, the optical storage medium interacting with the near-field probe beams to define corresponding near-field signal beams. In such a case, the detector includes a plurality of elements each responsive to optical energy, each near-field signal beam having a corresponding detector element, and the optics direct at least a portion of each near-field signal beam and a portion of the reference beam to interfere at the corresponding detector element.
In embodiments having the first and second detectors, the mask can include a plurality of apertures each having a dimension smaller than the wavelength of the input beam, wherein each aperture is configured to couple a portion of the measurement beam to the optical storage medium to define a near-field probe beam for the aperture, the optical storage medium interacting with the near-field probe beams to define corresponding near-field signal beams. The first and second detectors then each include a plurality of elements each responsive to optical energy, each near-field signal beam having a corresponding detector element for each of the first and second detectors, and the optics direct a first portion of each near-field signal beam and a first portion of the reference beam to interfere at the corresponding detector element for the first detector and direct a second portion of each near-field signal beam and a second portion of the reference beam to interfere at the corresponding detector element for the second detector.
The optics in the system can define a confocal imaging system.
The optics can include a spatial filter positioned before the detector, the spatial filter including a pinhole aligned with the detector element, and imaging optics positioned to image at least a portion of the near-field signal beam emerging from the aperture onto the pinhole.
The system can further include a stage for supporting the optical storage medium and at least one of a scanner and a stepper coupled to the stage for adjusting the position of the optical storage medium relative to the near-field probe beam. The system can further include an electronic processor coupled to the detector and the at least one of the scanner and the stepper, wherein during operation the electronic processor analyzes at least one signal generated by the detector element as a function of the relative stage position. Furthermore, the system can include a pulsed source which during operation generates the input beam, wherein the electronic processor is coupled to the pulse source to synchonize the stage adjustments.
The beam splitter and the mask in the system can be positioned to cause the measurement beam to contact the mask at substantially normal incidence. Alternatively, the beam splitter and the mask can be positioned to cause the measurement beam to contact the mask at an angle of incidence greater than 10xc2x0.
The aperture can be defined by a hole in the mask. Alternatively, the mask can include a first material having a first complex refractive index and a second material having a second complex refractive index different from the first complex refractive index, the second material defining the aperture. Also, the mask can include a waveguide defining the aperture. Also, the mask can include a first reflective material and a second dielectric material defining the aperture. Also, where the mask includes a scattering site, the mask can include a reflective first material and a second material having optical properties different from those of the first material, the second material defining the scattering site.
The system can further include a phase shifter positioned to shift the phase of the reference beam relative to the phase of the measurement beam. For example, the phase shifter can be positioned along the path of the reference beam. The system can further include an electronic processor coupled to the detector and the phase shifter, wherein during operation the electronic processor sets the phase shift imparted by the phase shifter to each of multiple values and analyzes a signal generated by the detector element for each of the multiple values. Also, the system can further include an electronic processor coupled to the detector and the phase shifter, wherein during operation the electronic processor causes the phase shift "khgr" imparted by the phase shifter to be modulated according to "khgr"="khgr"0+"khgr"1 cos xcfx89t, where "khgr"1≈0, t is time, and xcfx89 is the modulation frequency, and analyzes a signal generated by the detector element with respect to the modulation frequency.
The system can further include a source for generating the input beam. For example, the source can include a modulator producing a difference frequency xcfx89 between two components of the input beam. Also, the source may cause the two components of the input beam to have orthogonal polarizations. The system can further include an electronic processor coupled to the detector and the modulator, wherein the frequency difference xcfx89 produces a phase difference xcfx89t between the two components of the input beam, where t is time, and wherein the electronic processor analyzes a signal produced by the detector with respect to the phase difference xcfx89t. The source can be a pulsed source, wherein the electronic processor is coupled to the pulsed source to synchronize the signal analysis with the phase difference xcfx89t.
The system can further include a source for the input beam and the source can cause the input beam to have one of multiple wavelengths. Furthermore, the system can include an electronic processor coupled to the detector and the source, wherein the electronic processor analyzes a signal produced by the detector for each of the multiple wavelengths of the input beam.
The system can further include a retardation plate positioned along the path of the input beam and configured to adjustably control the polarization of the input beam. The system can further include an electronic processor coupled to the detector and the retardation plate and wherein during operation the electronic processor causes the retardation plate to impart each of multiple polarizations to the input beam and analyzes a signal generated by the detector element for each of the multiple polarizations.
The system can further include: a source for the input beam; an electromagnet positioned adjacent the optical storage medium; and a write beam source positioned to direct at least a portion of a write beam to the mask aperture, the mask aperture configured to couple at least a portion of the write beam to the optical storage medium to define a near-field write beam. In such a case, the optical storage medium can be a magneto-optic material, and the system can further include an electronic controller coupled to each of the electromagnet and the write beam source for controllably causing the reversal of a magneto-optic domain illuminated by the near-field write beam in the optical storage medium. The system can further include the optical storage medium. In some embodiments, the write beam source can adjustably occupy the detector position, thereby adjustably permitting the optics to direct the at least a portion of the write beam to the mask aperture. The near-field write beam can interfere with the near-field probe beam to cause the reversal of the magneto-optic domain.
In general, in another aspect, the invention features a method for reading information from an optical storage medium including: separating an input beam into a measurement beam and a reference beam; directing the measurement beam to a mask including at least one aperture having a dimension smaller than the wavelength of the input beam, wherein the mask aperture couples at least a portion of the measurement beam to the optical storage medium to define a near-field probe beam, the storage medium interacting with the near-field probe beam to define a near-field signal beam; and measuring optical interference between at least a portion of the reference beam and at least a portion of the near-field signal beam. The method can further include features corresponding to any of the features of the optical storage system described above.
In general, in another aspect, the invention features an optical storage system including: a write beam source providing at least one write beam; a reference beam source providing at least one reference beam; an optical storage medium; a mask having an aperture positioned to couple at least a portion of the at least one write beam and at least a portion of the at least one reference beam to the optical storage medium, the aperture having a dimension smaller than the wavelength of the at least one write beam; a confocal imaging system positioned to couple the at least one write beam and the at least one reference beam to the mask; and an electromagnet positioned adjacent the optical storage medium. The optical storage system can further include a phase shifter positioned to adjust the phase of the at least one reference beam relative to the at least one write beam. The mask can include multiple apertures each having a dimension smaller than the wavelength of the at least one write beam.
In general, in another aspect, the invention features an optical system including: a beam splitter positioned to separate an input beam into a measurement beam and a reference beam; a mask positioned to receive the measurement beam, the mask including at least one aperture having a dimension smaller than the wavelength of the input beam, wherein the mask aperture is configured to couple at least a portion of the measurement beam to a sample to define a near-field probe beam, the sample interacting with the near-field probe beam to define a near-field signal beam; a detector having an element responsive to optical energy; and optics positioned to direct at least a portion of the reference beam and at least a portion of the near-field signal beam to interfere at the detector element.
Embodiments of the invention may include any of the following advantages.
One advantage is that the interferometric analysis of the near-field signal beam can improve the signal-to-noise of the near-field information, e.g., the complex amplitudes of near-field beams scattered/reflected by a sample.
Another advantage is that the interferometric analysis can reveal changes in the phase or complex amplitude of near-field signal beams as a function of sample location.
Another advantage is that the confocal features of the systems and methods can remove background contributions from the signal of interest.
Another advantage is that the systems and methods can operate in a continuous scan mode with a pulsed input optical beam.
Another advantage is that in embodiments operating in a reflection mode, each mask aperture couples a near-field probe beam to the sample and couples a near-field signal beam toward the detector. Thus, each mask aperture is both a transmitter and receiver for a corresponding near-field beam, thereby improving lateral resolution. As a further result, the directions of propagation of the components of each near-field probe beam that produce a corresponding near-field signal beam at a given volume section of the sample are substantially the same, thereby simplifying an inverse calculation for properties of the sample using the complex amplitude of the near-field signal beam from the interference signal(s).
Another advantage is that the sample can be profiled using substantially low order electric and magnetic multipole near-field sources, e.g., near-field probe beam sources including an electric dipole and two different orthogonal orientations of a magnetic dipole.
Another advantage is that effects of interference terms caused by a background beam scattered and/or reflected from the mask apertures can be compensated. The interference terms can include interference between the background beam and the reference beam, and the background beam and the near-field signal beam.
Another advantage is that statistical errors in measured amplitudes and phases of the near-field signal beams can be substantially the same as statistical errors based on Poisson statistics of the reflected/scattered near-field probe beams. In other words, the measured amplitudes and phases are not significantly degraded by the presence of background signals.
Another advantage is that the sample properties can be analyzed by using multiple wavelengths.
Another advantage is that the separation between the mask and the sample can be varied to measure the radial dependence of the amplitudes and phases of the near-field signal beams.
Another advantage is that the relative lateral position of the mask and the sample can be varied to measure the angular dependence of the amplitudes and phases of the near-field signal beams.
Another advantage is that the spatial resolution of the system is defined primarily by the dimensions of the mask apertures and their distance from the sample, and is only weakly dependent on the optical system imaging the near-field signal beams emerging from the mask apertures onto the detector array.
Another advantage is that the sample scanning may be implemented in a xe2x80x9cstep and repeatxe2x80x9d mode or in a continuous scan mode.
Another advantage is that a source of the near-field probe beam may be a pulsed source, which may be synchronized with the sample scanning.
Another advantage is that by using a mask with an array of apertures, multiple interference terms can be measured substantially simultaneously for a one-dimensional or a two-dimensional array of locations on the sample. Furthermore, background noise in the multiple interference terms are correlated to one another.
Another advantage is that a given state of magnetization at the region of the sample illuminated by the near-field probe beam can be measured based on the polarization rotation of the near-field signal beam.
Another advantage is that the system can be used to write to an optical data storage medium such as a magneto-optical material.
Another advantage is that the system can profile a surface and internal layers near the surface of an object being profiled/imaged without contacting the object.
Another advantage is that either optical heterodyne or homodyne techniques may be used to measure amplitudes and phases of interference terms between the reference beam and the near-field signal beams.
Another advantage is that the complex refractive index of the sample at a location illuminated by the near-field probe beam can be determined from measured arrays of interference data corresponding to the near-field signal beams, wherein the dimensionality of the arrays may comprise one or two dimensions corresponding to one and two dimensions of space, a dimension for the spatial separation of the mask and the sample, a dimension for each of wavelength of components of the near-field probe beam source, and a dimension for the multipole characterization of the near-field probe beam.
Another advantage is that multiple layers of optical data stored on and/or in an optical storage medium can be read by measuring interference data for multiple separations between the mask and the sample.
Another advantage is that multiple layers of optical data stored on and/or in an optical storage medium can be read substantially simultaneously by measuring interference data for multiple wavelengths of the near-field probe beam, and/or different polarizations of the near-field probe beam.
Another advantage is that the mask can include sub-wavelength apertures in a sub-wavelength thick conducting layer, wavelength and sub-wavelength Fresnel zone plate(s), microlenses, and/or gratings to alter the properties of the near-field probe beam(s).
Another advantage is that a change in temperature of a site in or on the sample can be detected as a corresponding change in the complex value of the index of refraction.
Other aspects, features, and advantages follow.